The present invention relates to the compensation of time errors in a television signal, particularly a recorded signal.
A television signal coming from a recording instrument, e.g. on a magnetic tape or video record, is known to be subject to time errors resulting from fluctuations in speed or changes in the shape of the record carrier. Often the magnitude of these speed fluctuations or shape changes varies cyclically, causing the time error to vary in magnitude cyclically, at some frequency.
It is known to compensate such time errors by including an electronically controllable delay line in the signal path, as disclosed, for example, in German Pat. No. 1,014,153. A phase comparison of the line sync pulses separated from the faulty signal with constant line sync pulses or a comparison of an additionally recorded pilot carrier with a constant pilot carrier (e.g. 600 kHz) produces a setting value which represents the time error and which changes the delay time in the delay line on the basis of a fixed basic delay time to compensate the time error. If, for example, the line scanning period of the signal is too short, the delay time is increased and the signal is expanded relative to the time axis to the correct value of the line scanning period.
For such a circuit it is also known, as disclosed in German Offenlegungsschrift No. 2,122,592, and U.S. Pat. No. 3,863,022 based on application Ser. No. 251,229 filed by Walter Bruch on May 8th, 1972, to employ, as the controllable delay line, an electronic memory, such as a bucket cascade device, which is timed by clock pulses, and to influence the frequency of the clock pulses by the setting value.
It has now been found that when the above-described time errors occurring in the signal vary cyclically in amplitude at certain frequencies such a circuit does not achieve sufficient time error compensation. It was noted that the signal became even poorer when the frequency of the time error variations increased. This drawback could not be overcome by increasing the amplification of the setting value since then the errors would be overcompensated at other time error variation frequencies.